x86: make hypervisor build with gcc11

Gcc 11 looks to make incorrect assumptions about valid ranges that
pointers may be used for addressing when they are derived from e.g. a
plain constant. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100680.

Utilize RELOC_HIDE() to work around the issue, which for x86 manifests
in at least
- mpparse.c:efi_check_config(),
- tboot.c:tboot_probe(),
- tboot.c:tboot_gen_frametable_integrity(),
- x86_emulate.c:x86_emulate() (at -O2 only).
The last case is particularly odd not just because it only triggers at
higher optimization levels, but also because it only affects one of at
least three similar constructs. Various "note" diagnostics claim the
valid index range to be [0, 2⁶³-1].

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 722f59d38c
master date: 2021-05-27 14:40:29 +0200

tools/tests/x86_emulator/x86-emulate.c

@@ -2,6 +2,13 @@
 
 #include <sys/mman.h>
 
+/* See gcc bug 100680, but here don't bother making this version dependent. */
+#define gcc11_wrap(x) ({                  \
+    unsigned long x_;                     \
+    __asm__ ( "" : "=g" (x_) : "0" (x) ); \
+    (typeof(x))x_;                        \
+})
+
 #define cpu_has_amd_erratum(nr) 0
 #define cpu_has_mpx false
 #define read_bndcfgu() 0